Harmful immune response contributes to development of heart failure (HF). Similar harmful immune response may also induce cardiac diastolic dysfunction in sepsis that greatly increases mortality. Immune therapies for these two diseases have not shown notable success. In response, this R03 study will identify a new paradigm that a myofilament protein can modulate immune response to protect the heart. Cardiac myosin binding protein- C (cMyBP-C) is a regulatory protein that resides on the thick filament of the heart. Results from the parent K08 grant funded research has definitively shown that cMyBP-C is critical mediator of diastolic function. A recently completed prospective clinical study suggests that circulating cMyBP-C may confer cardiac protection. Majority (8/11) of cMyBP-C consists of immunoglobulin domains. A multiple sequences alignment program (COBALT) suggests that cMyBP-C may interact with CD74, which assists with antigen presentation and immune cell trafficking. Thus, the combination of immune response causing diastolic dysfunction, cMyBP-C mediating diastolic function, cMyBP-C providing cardiac protection, and homology with CD74 suggests that cMyBP-C can affect immune response. In an explorative study using lipopolysaccharide (LPS) to induce septic-like condition in cMyBP-C(-/-) and WT mice, the presence of cMyBP-C prevented development of diastolic dysfunction and recruited a probable population of protective immune cells to the heart. Thus, we hypothesize that cMyBP-C modulates immune response to protect heart function. However, due to inherent cardiac dysfunction of cMyBP- C(-/-), further clarification is needed. [Aim #1] We will determine contributions of cMyBP-C presence and cardiac dysfunction toward destructive immune response by: (1) using LPS to challenge cMyBP-C(-/-) mouse, predominantly diastolic dysfunction cMyBP-C(t3SA) mouse, and WT mouse with severe systolic dysfunction induced by trans-aortic constriction; (2) documenting changes in cardiac function with echocardiography; (3) quantifying and identifying leucocytes that filtrated the heart; and (4) determining immune modulatory function of heart-extracted leucocytes. [Aim #2] We will determine the underlying mechanism of immune cell mediated diastolic dysfunction by performing simultaneous measurements of force and intracellular calcium on intact WT papillary muscles that were incubated with heart-extracted leucocytes from LPS challenged cMyBP-C(-/-) and WT mice. This will identify contributions of changes in calcium handling and myofilament response toward diastolic dysfunction. Completion of this study will show: cMyBP-C modulation of immune response; immune cells that provide cardiac protection; and mechanisms by which immune cells cause diastolic dysfunction.